Modern electrical therapeutic and diagnostic devices for the heart, such as pacemakers, cardiovertors, and defibrillators, require a reliable electrical connection between the device and a region of the heart. Typically, a medical electrical lead is used for the desired electrical connection.
One type of commonly-used implantable lead is a transvenous lead generally taking the form of an elongated, substantially straight, flexible, insulated conductor. This type of lead is positioned through the venous system to attach to, and/or form an electrical connection with, the heart at the lead distal end. At the proximal end, the lead is typically connected to an implantable pulse generator. Because this type of lead may be placed through the venous system, electrical contact with the heart can be accomplished without requiring major thoracic surgery.
The specific design of a transvenous lead is generally dictated by the region of the heart in which it will be used. For example, U.S. Pat. No. 4,402,330 to Lindemans discloses a body implantable lead in which the lead body has a J-curve including a distal electrode with a permanent bend. This curve allows the lead to be readily positioned within, and connected to, the right atrium.
While the lead described in the '330 patent has been found acceptable for pacing the right atrium, a need exists for a similar transvenous medical electrical lead adapted for use in the left atrium. Such leads have been difficult to develop for a number of reasons. For example, minor blood clots are often caused by implanted objects placed within the vascular system. Should lead implantation cause blood clots to develop within the left side of the heart or associated vasculature, the direction of blood flow could cause these clots to be carried to the brain, causing stroke and other tissue damage. Thus, at present, chronic transvenous leads may not be safely implanted within the left side of the heart.
Despite the difficulties with lead placement, there remains a great need to be able to electrically stimulate and/or sense the left side of the heart since it accounts for the majority of the heart's hemodynamic output. For this reason, various pathologies may be better treated through stimulation on the left side of the heart. For example, in patients with dilated cardiomyopathy, electrical stimulation of both the right and left sides of the heart has been shown to be of major importance to improve the patient's well-being and to manage heart failure. See, for example, Cazeau et al., “Four Chamber Pacing in Dilated Cardiomyopathy,” PACE, November 1994, pgs. 1974-79. See also Brecker and Fontainem, St. et al., “Effects Of Dual Chamber Pacing With Short Atrioventricular Delay In Dilated Cardiomyopathy,” Lancet November 1992 Vol. 340 p1308-1312; Xiao H B et al., “Effect Of Left Bundle Branch Block On Diastolic Function In Dilated Cardiomyopathy,” Br. Heart J 1991, 66(6) p 443-447; and Fontaine G et al, “Electrophysiology Of Pseudofunction,” C. I. Meere (ed.) Cardiac pacing, state of the art 1979, Pacesymp, 1979 Montreal.
At present, there are several techniques for implanting a lead to the left side of the heart. For example, a median sternotomy, an intercostals approach, or, in a more limited procedure, a sub-xiphod approach may be used to place a lead on the external surface of the heart. These procedures, however, involve major surgery, which may be painful and dangerous for the patient, as well as extremely costly. The sub-xiphod approach, moreover, only permits limited access to the anterolateral surface of the left ventricle as well as to the left atrium.
An alternative approach involves electrically accessing the left atrium through the coronary sinus. Many catheter designs are available to facilitate lead placement in the coronary sinus. For example, U.S. Pat. No. 5,423,772 to Lurie, et. al. discloses a coronary sinus catheter having three sections. Each section has varying degrees of flexibility, with the proximal reinforced section being stiffer than an intermediate section, and the intermediate section being stiffer than a softened tip section. The catheter includes a curve extending from the intermediate section and continuing into the softened tip section, where the radius of curvature decreases. One drawback to such a design, however, is that the particular shape of the curve is not ideally suited for electrically accessing the left atrium. In addition, such a catheter is relatively complicated to manufacture because of the braid or other means that is required to reinforce the proximal section. Finally, such a catheter does not permit introduction of a stylet to assist in the placement of the catheter into the coronary sinus.
Another design is disclosed in U.S. Pat. No. 6,161,029 to Spreigl, et al. This design utilizes a balloon-expandable or self-expanding stent-like electrode that is deployed within the coronary sinus to distribute the electrode surface area over a wide area and to maintain the distal lead end in place. However, this type of lead is difficult to re-position or remove, as may be necessary to improve thresholds, to increase intrinsic signal amplitudes, or to replace the lead in the case of chronic problems such as lead failure or infection.
Yet another lead design is discussed in U.S. Pat. No. 6,006,122 to Smits. The disclosed lead utilizes a bent fixation ring positioned adjacent to a distal coronary sinus electrode. The ring, which is formed of a pliable material, is adapted to wedge or fix the lead within the coronary sinus in such a manner that the electrode is pushed against the vessel wall without impeding blood flow through the vessel. One of the drawbacks of this particular design is the inability to re-position and/or remove the electrode as may be required for any of the reasons discussed above.
U.S. Pat. No. 5,129,394 to Mehra describes a method and apparatus for sensing in vivo blood pressure proportional to the left ventricular pressure for detecting ventricular tachyarrhythmias or the cardiovascular status in congestive heart failure, and/or for adjusting the rate of a pacemaker. A lead with a pressure sensor near its distal end is placed transvenously through the coronary sinus and located in the coronary vein.
When in place, an inflatable balloon proximal to the pressure sensor may be used to acutely occlude the coronary vein until the sensor position is stabilized by the growth of fibrous tissue. According to this mechanism, the sensor may not be used for approximately six weeks until fibrous tissue has formed. After that, the lead may not be easily re-positioned or removed.
Other types of lead systems employ a shape memory-metal or other super elastic material designed to make the leads easier to deploy and affix. For example, U.S. Pat. No. 4,913,147 to Fahlstrom, et. al. describes a lead including one or more components formed of a shape-memory metal. These components are designed to have a first shape when at body temperature, and a second shape when at a different predetermined temperature. Such a component may be disposed at the distal lead end to assist in providing a reliable mechanical and electrical connection to the heart when the component changes shape. For example, this type of component may be disposed in proximity to the electrode to assume a first shape permitting easy introduction of the lead through a vein, and a second shape such as a curve that is adapted to maintain the electrode at a predetermined position within the heart or vascular system. In one disclosed embodiment of the device, the lead includes an extendable, non-expanding helix that remains smaller than the inner diameter of the lead lumen following deployment.
U.S. Pat. No. 5,522,876 to Rusink describes a lead for use with a pacemaker in a pacing system, the lead having at least one electrode and a helical fixation member at the lead distal tip. The helical member, which is adapted to be affixed to heart tissue, is composed of shape-memory metal. The helix is encapsulated within a mannitol or other dissolvable member in a shrunken state so that the helix diameter is less than the diameter of the lead casing. When the dissolvable member is dissolved by body fluids, the helix is released to assume an expanded diameter that is greater than the electrode diameter. When the helix is embedded into the heart wall, the helical coils are displaced radially away from the outer edge of the tip electrode so that the damage to the heart tissue immediately proximate to the tip electrode is minimized. One disadvantage of this system is that the helix is not retractable once it is deployed. Moreover, the design is adapted for use in the right ventricular or right atrial cardiac wall.
What is needed, therefore, is an improved lead adapted for use in the coronary sinus, middle and great cardiac veins, or another vessel that is both easy to deploy, and that may be readily removed and/or re-positioned.
It is thus an object of the present invention to provide a medical electrical lead that is suitably shaped to provide an electrical connection through the coronary sinus to one or both of the left chambers of the heart.
A still further object of the present invention is to provide a medical electrical lead having an electrode positioned so that when the lead is implanted into the coronary sinus, the electrode is positioned against the coronary sinus wall.
A still further object of the present invention is to provide a medical lead having a fixation method that may be extended and retracted to allow positioning and re-positioning of the lead.
A still further object of the present invention is to provide a medical lead having a fixation helix constructed of shape memory metal or other super elastic material that, upon extension, increases in diameter to the vessel wall, securing the lead in position.
A still further object of the present invention is to provide a medical electrical lead having an electrode that may be positioned along a selected portion of the coronary sinus wall in a manner that minimizes the restriction of blood flow through the coronary sinus.